Embodiments of the present invention relate generally to time and frequency alignment systems operating over packet-switched communications networks and, more specifically, to methods and apparatus for providing a distributed boundary clock function involving book-end units when there is a dedicated communication channel that is not packet-based between the two book-end units.
Packet-based timing methods are becoming essential for delivering timing over packet-switched networks, often referred to as the cloud. In particular, Precision Timing Protocol (PTP) (aka IEEE™ 1588-2008) is becoming popular for delivering timing information (time/phase/frequency) from a Grand Master (GM) clock to slave clocks in end application-specific equipment. For example, wireless base stations providing mobile telephony services require precise timing and the backhaul method of choice is Ethernet.
The Grand Master clock provides timing information over the packet-switched network to the slave clocks by exchanging packets with embedded time-stamps related to the time-of-arrival and time-of-departure of the timing packets. The slave clock utilizes this information to align its time (and frequency) with the Grand master. The Grand Master is provided an external reference to serve as the basis for time and frequency. Most commonly this reference is derived from a Global Navigation Satellite System (GNSS) such as the GPS System that in turn is controlled by the US Department of Defense and its timing controlled very precisely and linked to the US Naval Observatory. Time alignment to the GPS clock is, for all practical purposes equivalent to time alignment to UTC.
The communication path between the GM and the end-point slave unit may involve multiple switches and transmission links. Since packet-switching is statistical in nature, the transit delay of packets between GM and end-point slave is not constant, a phenomenon referred to as transit delay variation (TDV) or packet delay variation (PDV). This PDV acts like an additive noise to the timing signal and must be filtered out by the slave clock recovery algorithm. In order to allow for precise synchronization, IEEE 1588 proposes the notion of on-path timing support. The notion of full on-path support is when each and every switch between the GM and the end-point switch provides on-path support. On-path support in this scenario is achieved by boundary clocks (BCs) and transparent clocks (TCs). Full on-path support implies that every switch between the master and slave is either a boundary clock or a transparent clock. Note that for this description the terms switch and router are equivalent.
The transparent clock function represents the attempt by the switch to remove any transit delay variation introduced by the switch itself. The PTP packet structure provides “correction fields” that are updated by each TC to reflect the delay of that packet through the switch. This makes the switch nominally transparent to the timing packet flow from the viewpoint of timing noise or PDV.
A boundary clock performs a clock regeneration function. A boundary clock includes a slave side that “looks” towards the GM to develop a local clock that is synchronized to the GM. It has a master side that looks towards the end-point slave and performs a PTP master function using the clock recovered by the slave side as its reference for time.
The notion of partial on-path support is when not all the intermediate elements are PTP aware. The PTP-aware elements in this case are generally boundary clocks.
There are some situations where the link between two network elements is not packet based. One example of such a situation is when microwave radio is used to implement the transmission of Ethernet signals in a “backhaul” application. For example, wireless (telephony) base-stations may expect Ethernet connectivity back into the core telecommunications network but rather than deploy wired communication media, such as fiber, microwave radios are employed. Such radios operate in “book-end” fashion whereby a segment of transmission has two radio TX/RX pairs emulating a direct connection. Such TX/RX pairs have, in addition to the link for Ethernet traffic, a dedicated low-speed communication link between the two that is often called a “wayside channel”. This low-speed channel is often an E1 or DS1 channel. For providing on-path support, these two ends of the segment must collaborate to form a distributed boundary clock wherein the slave side of the BC is at one end and the master side of the BC is at the other. This implies that a method for transferring the time/phase of the BC-slave to the BC-master is required.
The invention described here provides a method to transfer timing over the dedicated low-speed link, permitting the two book-ends to collaborate and function as a distributed boundary clock.